Ceramic body and batch for making



Jan 10, 1950 c. R. AUSTIN ET AL 2,494,277

CERAMIC BODY AND BATCH FOR MAKING SAME Filed Nov. 16, 1944 INVENTOR? Chesfzr R Ausf/n.

B I Edwin J P0 zrs M/WWW kw rim TTORNEYS Patented Jan. 10, 1950 l I 2,494,277 CERAMIC BODY AND ng'rcn FOR MAKING SAM Chester R. Austin, Columbus, Ohio, and Edwin J. Rogers, Troy, Y., assignors, by mesne assigne Hartford, Conn., a corporation of Delaware, and Ethyl Corporation, Detroit, Mich, a corporation of Delaware Application November 16, 1944, Serial No. 563,666

3 Claims. (Cl. 106 -63) The present invention relates to ceramic bodies. expansion, and good electrical insulating proper- Mo espe ally it s c n e n d W th the producties and resistance to attack by lead compounds tion of an electrical insulating material that is at elevated temperatures. Particularly p ed f r p rk-plug insulators A further object of our invention is to produce and for uses requiring similar qualities. 5 such an insulator by a careful selection of in- The increasing use of ceramic insulators in airgredients to insure that losses occurring through craft engines has brought about a demand for warpage during firing are maintained at a minispark-plug insulators of a superior quality. Pormum.

celain spark-plug insulators were among the Another object of our invention is to provide, first used in internal combustion engines. Later, by careful selection of ingredients, an aluminabodies of the mullite type were developed The ase ceramic body in which the alumina can be present demand for the best possible materials re-crystallized at temperatures below 3000 F has brought about the development of bodies of A further obJect of our invention is to provide were made of almost pure alumina; however, in ties are maintained at elevated temperatures even such cases extremely high burning temperatures though an appreciable amount of alkalies may of around 3200 F. or higher are required to efiect be present. the rel-crystallization that is necessary to produce It is well known that various fluxes are effecthe proper characteristics. Such high temperative to lower materially the re-crystallization tures exceed the limits of most commercial kilns temperature of alumina bodies. Such materials and, therefore, introduce serious production as MgO, BaO, CaO, F6203, and SiO2 have been problems. Mica plugs are unsatisfactory for pro osed as fluxes for this purpose; however, high-output engines. none of these fluxes yields an insulator having all The ideal spark-plug insulator must be resistof the desired characteristics. If sufiicient magant to thermal shock in order to withstand the nesia is added to lower the re-crystallization tem- High thermal expansion is also desirable to instrength, Calcium oxide, when added in amounts as possible with the metal parts with which it the desired range, tends to produce bodies having is assembled. In addition, of course, it must an undesirably short burning range, a rather maintain its electrical insulating properties at coarse crystal structure, and a definite tendency the elevated temperatures to which it is subjected to warp badly during firing. The addition of most important characteristics of the ideal insu- Silica, when present in amounts adequate to lower produced during assembly and the stresses that and resistance to thermal shock. An increase in arise during service. the silica content also lowers the resistance of the From a manufacturing standpoint, it is desirbody to attack by lead oxide.

able that the ceramic body of the insulator retain The above and other objects and advantages its form and symmetry and that it ave a uniof our invention will appear in the following deform shrinkage This uniformity makes possible scription and appended claims when considered the production of an article having close dimenin conJunction with the accompanying drawing sional tolerances. In addition, the body should which forms a part. of this specification.

mature at a temperature below 3000 F. so that In said drawing the single figure shows a reprothe insulators may be burned in commercially duction of a photomicrograph of a typical thinavailable kilns. section of a body composed primarily of alumina It is, therefore, one of the objects of our inand containing small amounts of calcium oxide,

sparkeplug insulators having high mechanical invention. strength, high thermal conductivity, highthermal Before explaining in detail the present invencially true when bodies between 91 to 98.5 per referred to herein will percentages by weight.

the ceramic bodies of clude small amounts example, we have found that ceramic bodies hav- 'ing excellent tion, it is to be understood that the invention is not limited in its application to the details illustrated in the accompanying drawing, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation, and it is not intended to limit the invention herein claimed beyond the requirements of the prior art.

We have found, and have disclosed and claimed in our copending application Ceramic body, Serial No. 563,664, filed November 16, 1944, now Patent Number 2,423,958, that alumina bodies having the requisite properties for spark-plug insulators and other ceramic applications, can be produced by the addition of the proper amount of calcium oxide (preferably added as a calcium oxide-bearing material) and silica. We have found further and have disclosed and claimed in the aforementioned copending application, that it is particularly advantageous and desirable to introduce the calcium oxide into the body as tricalcium penta-alum'inate. This latter is espethe tri-calcium penta-aluminate is fused and allowed to crystallize corporation in the body. Some of these bodies have compressive strengths as high as 240,000 pounds per square inch.

We have now discovered that even higher strengths can be obtained by incorporating in the alumina body small amounts of calcium oxide, silica. and boric oxide. Again, the best results are obtained when the calcium oxide is added as tri-calcium penta-aluminate.

As mentioned hereinbefore, we have discovered that particularly desirable properties may be obtained in ceramic bodies composed essentially of alumina, calcium oxide. silica. and boric oxide. In general. we have found that the best results are obtained with bodies containing from 0.25 to 3.0 oer cent calcium oxide. from 0.25 to 5.0 per cent si ica. and from 0.25 to 5.0 per cent boric oxide. the balance of the body being essentially alumina. However. we prefer to use, for the highest stren ths and for the greatest ease of handling, from 0.5 to 2.0 per cent calcium oxide,

from 0.5 to 3.0 er cent silica, and from 0.5 to 3.0

'per cent of boric oxide, with the balance of the body essentially alumina.

In other words, the ceramic bodies of this invention contain, in general, at least about 87 per ,cent of alumina, in excess of about 0.25 per cent of calciumoxide, more than about 0.25 per cent of silica. and in excess of about 0.25 per cent of boric oxide.

For the most commercially attractive bodies, we prefer to keep the alumina content of the cent, with from 0.5 to 3.0 per cent calcium oxide, from 0.5 to 3.0 per cent silica, and from 0.5 to 3.0 per cent boric oxide. Unless otherwise specified, all percentages be understood to represent Although only four components have been specifically mentioned in the foregoing discussion,

our invention may also inof other substances. For

properties may be produced even though the alumina contains as much as 0.5

or 0.6 per cent alkali, expressed as NazO. Also,

we have found that the calcium oxide content prior to its inbe obtained from a relatively calcium carbonate or, if desired, from a limestone which may contain some desirable, however,

pure high-calcium incidental impurities. that the four primary the proportions given other non-essential ingredients may In general, we prefer to keep the amounts of the non-essential ingredients as low as possible.

As a source of alumina, we may use any rela tively pure grade; however, we prefer to use one of the various grades manufactured by the Bayer process. We have found that the best results are obtained when the alumina is calcined, preferably at a temperature in excess of 2000 F.,. prior to use in the body. As previously discussed, the alumina used in the ceramic bodies of our in-- vention may contain as much as 0.5 or 0.6 per cent of alkali, expressed as 'NacO, without de stroying the high electrical insulating properties at elevated temperatures.

Although, as mentioned above, the calcium oxide may be added to the batch as calcium oxide or in any form, such as calcium carbonate or highlime limestone, that will decompose during processing to yield calcium oxide, we prefer to add the calcium oxide to the body as tri-calcium penta-aluminate, and we have found that it is decidedly advantageous if the tri-calcium pentaaluminate is fused and allowed to crystallize prior to its incorporation in the batch.

In the preparation of fused tri-calcium pentaaluminate, we prefer that calcium carbonate or a high-calcium quicklime be used as the source of calcium oxide and that Bayer process alumina preferably be used as the source of alumina. These materials, combined in the proper proportions, are then sintered or fused in an electric are or in any furnace capable of producing the temperature to effect combination, for example, approximately 3200 F.

We have used, for example, the following batch composition for the production of tri-calcium' penta-aluminate by fusion in a direct-arc electric furnace: 26.9 per cent high-calcium quicklime (pebble) and 13.1 per cent Bayer alumina (calcined). The quicklime used in this batch contained 89.5 per cent calcium oxide, and the alumina contained practically 100 per cent alumina. Other sources of calcium oxide and alumina may be used; however, in any combination, the ingredients are proportioned to give an approximate molecular ratio of 3:5 for the calcium oxide and the alumina, respectively.

Chemical analyses of the solidified material from representative fusions gave the results shown in Table 1.

TABLE 1 used to prepare samples for chemical analysis.

1 Silica over about 0.9 per cent was introduced in grinding operatloi 'mately 90 than 325-mesh.

samples indicates that they contain approxiper cent of tri-calcium penta-aluminate and from to per cent of mono-calcium aluminate, the balance being essentially a mixed mass of isolated crystals and glass.

ground so that a substantial amount will pass a 325-mesh sieve. For most purposes, we prefer that at least 90 per cent of the material is less be ground separately, either dry or wet, so that a substantial portion, preferably at least 99 per it for use in fabrication.

The prepared batch is then schedule produces f hours, and cool in place.

' were prepared as outlined above and were burned 6 shape and the intended application of the article being produced and with the amount and type of be judged by immersion of the product in an alcohol-fuchsine dye solution at approximately 1000 poundsper square inch pressure for about hour. The dye penetrates the porous areas, thereby disclosing their location. We have found that the following burning porcelain balls tends to the materials.

introduce some silica into It will be understood, of course,

dueed separately, as pick-up during grinding, as impurities in the raw materials used, or as a combination of two or more sources.

regulated by the amount of silica added, to the raw batch.

The effect of varying the percentages of the various components of our ceramic body is shown in the following tables. All of the bodies recorded F. in 14 hours, held at that temperature 4 hours, and cooled in place. In the case of bodies having appreciable porosity, no compressive strength determinations were made.

a porcelain mill. the bodies tend to become As these data show, porous when 4.6 per is incorporated.

Table 4 illustrates the properties obtained by Efiect of varying B20: contents in ceramic bodies prepared from a mimtureof alumina,

silica, tri-calcium penta-aluminate, and boric acid Batch Composition, Per Cent aff g fgig Burned Batch Dye A1103 3C80.5A1z03 Slog From the foregoing description of our invention, it will be apparent that we have provided a novel ceramic body having a novel structure. This body has an exceptionally high mechanical strength, good electrical insulating properties, good thermal conductivity, and excellent resistance to spalling and thermal shock. It is HsBOa A110; 030 Slog Pene- Linear Compressive tration B10 Shrinkage, Strength,

Per Cent Lbs/Sq. In.

result of the fact that the partly bonded also resistant by burning at temperatures below 3000 F.

Although we have indicated that the properto 3% silica, and 0.25 to 5% boron ceramic body having an analysis alumina, 0.5 to 3% calcium oxide, silica, and 0.5 to 3% boron trioxide. CHESTER R. AUSTIN. EDWIN J. ROGERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Caesar Number Date Apr. 2, 1878 Certificate of Correction Patent No. 2,494,271 January 10, 1950 CHESTER R. AUSTIN ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Columns 7 and 8, Table 3, line 1 of the title, for SiO, read 0110; columns 9 and 10, Table 5, sixth column thereof, for A1 0, read Algoa;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant C'omsnisaiomr of Patents. 

1. A BATCH FOR THE PRODUCTION OF A CERAMIC BODY, WHICH CONTAINS ABOUT 88% ALUMINA, ABOUT 10% TRI-CALCIUM PENTA-ALUMINATE, BETWEEN 1 TO 2% SILICA, AND ABOUT 1% BORIC ACID. 